Report from the fifth National Cancer Institute Mouse Models of Human Cancers Consortium Nervous System Tumors Workshop

The landscape of neuro-oncology in East Africa: a review of published records

INTRODUCTION

Primary central nervous system (CNS) tumors represent an important and growing cause of worldwide morbidity and mortality. There are global variations in the reported case burden of CNS tumors, with high-income countries reporting a higher incidence of cases than low- and middle-income countries. Variations are attributed to differences in access to care, diagnostic capacity, risk exposure, and under-reporting in LMICs. This study aims to review existing literature on the distribution of primary CNS tumors and neuro-oncologic care, and the contribution of scientists and institutions to neuro-oncologic research across 18 East African countries over the last 5 decades.

METHOD

A search was conducted using OVID Medline and PubMed databases to identify relevant East African neuro-oncologic studies published over the last 50 years.

RESULTS

The authors reviewed 36 neuro-oncology articles published across 8 of 18 East African countries. Kenya represented the highest number of published articles; ten countries queried yielded zero publications. A total of 2006 cases from all age groups were represented in published literature consisting of a wide spectrum of CNS tumors. One-third of reported cases were pediatric. Meningioma formed the largest proportion (43.3%) followed by glioma (33.7%). Sixty-seven percent of publications gave an overview of clinical care received by patients with most patients not receiving comprehensive neuro-oncologic care.

CONCLUSION

The modest collection of neuro-oncology publications from East Africa shows that the case diversity of primary CNS tumors in East Africa is comparable to the rest of the world. There is, however, poorer access to neurosurgical care and adjuvant therapy. Multidisciplinary efforts from clinicians, researchers, and healthcare agencies are needed to quantify and address the requisite neuro-oncology needs in this region.

Overcoming the Blood–Brain Barrier. Challenges and Tricks for CNS Drug Delivery

Treatment of certain central nervous system disorders, including different types of cerebral malignancies, is limited by traditional oral or systemic administrations of therapeutic drugs due to possible serious side effects and/or lack of the brain penetration and, therefore, the efficacy of the drugs is diminished. During the last decade, several new technologies were developed to overcome barrier properties of cerebral capillaries. This review gives a short overview of the structural elements and anatomical features of the blood–brain barrier. The various in vitro (static and dynamic), in vivo (microdialysis), and in situ (brain perfusion) blood–brain barrier models are also presented. The drug formulations and administration options to deliver molecules effectively to the central nervous system (CNS) are presented. Nanocarriers, nanoparticles (lipid, polymeric, magnetic, gold, and carbon based nanoparticles, dendrimers, etc.), viral and peptid vectors and shuttles, sonoporation and microbubbles are briefly shown. The modulation of receptors and efflux transporters in the cell membrane can also be an effective approach to enhance brain exposure to therapeutic compounds. Intranasal administration is a noninvasive delivery route to bypass the blood–brain barrier, while direct brain administration is an invasive mode to target the brain region with therapeutic drug concentrations locally. Nowadays, both technological and mechanistic tools are available to assist in overcoming the blood–brain barrier. With these techniques more effective and even safer drugs can be developed for the treatment of devastating brain disorders.

Therapeutic Hypothesis Testing With Rodent Brain Tumor Models

The development and application of rodent models for preclinical testing of novel therapeutics and approaches for treating brain tumors has been a mainstay of neuro-oncology preclinical research for decades, and is likely to remain so into the foreseeable future. These models serve as an important point of entry for analyzing the potential efficacy of experimental therapies that are being considered for clinical trial evaluation. Although rodent brain tumor models have seen substantial change, particularly since the introduction of genetically engineered mouse models, certain principles associated with the use of these models for therapeutic testing are enduring, and form the basis for this review. Here we discuss the most common rodent brain tumor models while directing specific attention to their usefulness in preclinical evaluation of experimental therapies. These models include genetically engineered mice that spontaneously or inducibly develop brain tumors; syngeneic rodent models in which cultured tumor cells are engrafted into the same strain of rodent from which they were derived; and patient-derived xenograft models in which human tumor cells are engrafted in immunocompromised rodents. The emphasis of this review is directed to the latter.

Contemporary murine models in preclinical astrocytoma drug development

Despite 6 decades of research, only 3 drugs have been approved for astrocytomas, the most common malignant primary brain tumors. However, clinical drug development is accelerating with the transition from empirical, cytotoxic therapy to precision, targeted medicine. Preclinical animal model studies are critical for prioritizing drug candidates for clinical development and, ultimately, for their regulatory approval. For decades, only murine models with established tumor cell lines were available for such studies. However, these poorly represent the genomic and biological properties of human astrocytomas, and their preclinical use fails to accurately predict efficacy in clinical trials. Newer models developed over the last 2 decades, including patient-derived xenografts, genetically engineered mice, and genetically engineered cells purified from human brains, more faithfully phenocopy the genomics and biology of human astrocytomas. Harnessing the unique benefits of these models will be required to identify drug targets, define combination therapies that circumvent inherent and acquired resistance mechanisms, and develop molecular biomarkers predictive of drug response and resistance. With increasing recognition of the molecular heterogeneity of astrocytomas, employing multiple, contemporary models in preclinical drug studies promises to increase the efficiency of drug development for specific, molecularly defined subsets of tumors.

Molecular mechanisms promoting the pathogenesis of Schwann cell neoplasms

Neurofibromas, schwannomas and malignant peripheral nerve sheath tumors (MPNSTs) all arise from the Schwann cell lineage. Despite their common origin, these tumor types have distinct pathologies and clinical behaviors; a growing body of evidence indicates that they also arise via distinct pathogenic mechanisms. Identification of the genes that are mutated in genetic diseases characterized by the development of either neurofibromas and MPNSTs [neurofibromatosis type 1 (NF1)] or schwannomas [neurofibromatosis type 2 (NF2), schwannomatosis and Carney complex type 1] has greatly advanced our understanding of these mechanisms. The development of genetically engineered mice with ablation of NF1, NF2, SMARCB1/INI1 or PRKAR1A has confirmed the key role these genes play in peripheral nerve sheath tumorigenesis. Establishing the functions of the NF1, NF2, SMARCB1/INI1 and PRKAR1A gene products has led to the identification of key cytoplasmic signaling pathways promoting Schwann cell neoplasia and identified new therapeutic targets. Analyses of human neoplasms and genetically engineered mouse models have established that interactions with other tumor suppressors such as TP53 and CDKN2A promote neurofibroma-MPNST progression and indicate that intratumoral interactions between neoplastic and non-neoplastic cell types play an essential role in peripheral nerve sheath tumorigenesis. Recent advances have also provided new insights into the identity of the neural crest-derived populations that give rise to different types of peripheral nerve sheath tumors. Based on these findings, we now have an initial outline of the molecular mechanisms driving the pathogenesis of neurofibromas, MPNSTs and schwannomas. However, this improved understanding in turn raises a host of intriguing new questions.